Evaluating the suppression of Hydrilla verticillata by manual removal and planting natives for small-scale restoration efforts in a spring-fed river

Abstract

Hydrilla (Hydrilla verticillata) is an invasive aquatic macrophyte found on every continent except Antarctica. Due to the high number of federally listed species in the San Marcos River, Texas, management options for hydrilla are limited. We examined the ability of the two native macrophytes to suppress hydrilla in plots where 0 to 100% hydrilla was removed. In plots where 100% hydrilla was removed, Texas wild rice exhibited equal or greater coverage to hydrilla. Hydrilla was present in 100% of the plots at the end of the study, regardless of percent removal, while Texas wild rice (Zizania texana) and water stargrass (Heteranthera dubia) were present in 50 and 42% of the plots, respectively. While the overall survival percentage in plots was 50% for Texas wild rice, the mean root and shoot dry weights were significantly greater than hydrilla which occurred in all plots. However, hydrilla coverage was among the highest in plots where 100% of the hydrilla was removed at the start of the study, indicating its ability to quickly invade disturbed sites. The study results indicate morphological differences with hydrilla allocating greater biomass into its shoots while Texas wild rice allocates equal amounts of biomass into its shoots and roots.